1. Computing and SE II Chapter 6: Software Design
Er-Yu Ding
Software Institute, NJU

2. Main Contents
What is Software Design?

3. What is Software Design? ----Characteristics of design situations (1)
Design situations start with a need and require intention
A need so identified acts as the initial motivational force that provides the basis for starting design work.
Design situations involve transformation
Design is the restructuring of a current situation to achieve some preferred situation
Generation of new ideas is fundamental to design situations
Design occurs whenever there is an “imaginative jump from present facts to future possibilities.”
The precise manner in which new ideas are generated cannot be codified.

4. What is Software Design? ----Characteristics of design situations (2)
Constraint satisfaction
An initial need determines the most basic constraints and requirements on a design situation.
In general, more constraints are eventually discovered during the design work itself.
The constraints that apply both to the designed artifact and to the processes and participants involved during the design activity.
Problem solving or decision making
The solution space for design problems is very large and its sheer size eliminates exhaustive search as a possible problem solving technique
“design” is characterized by a series of decisions between various design alternatives
Each divergent perspective may influence the progress of the design in different and unpredictable ways.

5. 1. What is Software Design
1. What is Software Design? ----Characteristics of design situations (3)
Design results in a scheme for implementing an artifact
“Design” is essentially “the formulation of a prescription or model for a finished work in advance of its embodiment”
Design representation serves as the basis to conceptualize and compare various design decisions.
Sometimes, a design does not result in a distinct “plan-then-implement” situation.
Often the design output occurs incrementally while the design and the artifact evolve together.

6. 1. What is Software Design
1. What is Software Design? Characteristics of design situations (4)
Diversity and evolution
Any particular design situation could be drawn in many different directions.
The designer’s act of making decisions among the various identified design alternatives ties together this evolution of the design.
The evolution of a design is often closely linked to the consolidation of the constraints and requirements applied in a particular design situation.

7. 1. What is Software Design? ----Characteristics of Software Design (1)
Design situations start with a need and require intention
RE is needed before software design
Finding the problems, defining the requirements
Design situations involve transformation
Software changes the world
Designing base on limited objects (shared phenomenon共享现象) and fixed rules (domain characteristics领域特性)
So, Requirements, Shared Phenomenon and Domain Characteristics comprising the basis of software design, that is Analysis Model

8. Analysis Model -> Design Model

9. 1. What is Software Design? ----Characteristics of Software Design (2)
Generation of new ideas is fundamental to design situations
Styles of idea generation in Software Design
Abstract
Elaboration
Modularization
Information Hiding
Encapsulation
Outcomes of idea generation
Functional independent (Separating concerns)
General Styles of Design Theory
Specific Styles of Software Design

10. 1. What is Software Design? ----Characteristics of Software Design (2)
Abstraction
Abstraction manages complexity by emphasizing essential characteristics and suppressing implementation details
Procedural abstract
Data abstract
Allows postponement of certain design decisions that occur at various levels of analysis, e.g.,
Interface and Implementation
Inherit

11. 1. What is Software Design? ----Characteristics of Software Design (2)
Elaboration
Stepwise Refinement
Decomposition
Hierarchical Structures
open
walk to door;
reach for knob;
open door;
repeat until door opens
turn knob clockwise;
walk through;
if knob doesn't turn, then
close door.
take key out;
find correct key;
insert in lock;
endif
pull/push door
move out of way;
end repeat

12. 1. What is Software Design? ----Characteristics of Software Design (2)
Modularization
High Cohesion: A module should encapsulate some well-defined, coherent piece of functionality (more on that later)
Low Coupling: Minimize the amount of dependencies between modules

13. 1. What is Software Design? ----Characteristics of Software Design (2)
Information Hiding
Information hiding is an important means of achieving abstraction
i.e., design decisions that are subject to change should be hidden behind abstract interfaces
Application software should communicate only through well-defined interfaces
Each interface should be specified by as little information as possible
If internal details change, clients should be minimally affected
May not even require recompilation and relinking...

14. 1. What is Software Design? ----Characteristics of Software Design (3)
Constraint satisfaction
Managing Software Quality, especially nonfunctional requirements

15. 1. What is Software Design? ----Characteristics of Software Design (3)
Software Quality
Functional and nonfunctional
Visible and invisible
Quality Guidelines
A design should exhibit an architecture
A design should be modular
A design should contain distinct representations
A design should lead to data structures that are appropriate.
A design should lead to components that exhibit independent functional characteristics.
A design should lead to interfaces that reduce the complexity.
A design should be derived using a repeatable method.
A design should be represented using a notation that effectively communicates its meaning.

16. 1. What is Software Design? ----Characteristics of Software Design (4)
Problem solving or decision making
Emphasis on fundamental decision
Architectures Design
Dividing decisions making work with different concerns
HCI Design
Distribution Design
Safety Design
Real-Time Design …

17. 1. What is Software Design? ----Characteristics of Software Design (4)
Architectures
“The overall structure of the software and the ways in which that structure provides conceptual integrity for a system.” [SHA95a]
Structural properties. This aspect of the architectural design representation defines the components of a system (e.g., modules, objects, filters) and the manner in which those components are packaged and interact with one another. For example, objects are packaged to encapsulate both data and the processing that manipulates the data and interact via the invocation of methods
Extra-functional properties. The architectural design description should address how the design architecture achieves requirements for performance, capacity, reliability, security, adaptability, and other system characteristics.
Families of related systems. The architectural design should draw upon repeatable patterns that are commonly encountered in the design of families of similar systems. In essence, the design should have the ability to reuse architectural building blocks.

18. 1. What is Software Design? ----Characteristics of Software Design (5)
Design results in a scheme for implementing an artifact
Software Model: Design Model

19. The Design Model

20. 1. What is Software Design? ----Characteristics of Software Design (5)
Design Model Elements
Data elements
Data model --> data structures
Data model --> database architecture
Architectural elements
Application domain
Analysis classes, their relationships, collaborations and behaviors are transformed into design realizations
Patterns and “styles”

21. 1. What is Software Design? ----Characteristics of Software Design (5)
Design Model Elements
Interface elements
the user interface (UI)
external interfaces to other systems, devices, networks or other producers or consumers of information
internal interfaces between various design components.
Component elements
Local data structures, behaviors and algorithms
Deployment elements
Environment, Distribution

22. 1. What is Software Design? ----Characteristics of Software Design (6)
Diversity and evolution
Sharing common knowledge of
Problems Solving: Patterns
Architecture styles
Design Patterns
Coding Patterns: data structures
Software evolution: Refactoring
Reusing successful solution
Frameworks
Components (构件)

23. 1. What is Software Design? ----Characteristics of Software Design (6)
Design Patterns
The best designers in any field have an uncanny ability to see patterns that characterize a problem and corresponding patterns that can be combined to create a solution
A description of a design pattern may also consider a set of design forces.
Design forces describe non-functional requirements (e.g., ease of maintainability, portability) associated the software for which the pattern is to be applied.
The pattern characteristics (classes, responsibilities, and collaborations) indicate the attributes of the design that may be adjusted to enable the pattern to accommodate a variety of problems.

24. 1. What is Software Design? ----Characteristics of Software Design (6)
Refactoring
Fowler [FOW99] defines refactoring in the following manner:
"Refactoring is the process of changing a software system in such a way that it does not alter the external behavior of the code [design] yet improves its internal structure.”
When software is refactored, the existing design is examined for
redundancy
unused design elements
inefficient or unnecessary algorithms
poorly constructed or inappropriate data structures
or any other design failure that can be corrected to yield a better design.

25. 1. What is Software Design? ----Characteristics of Software Design (6)
Frameworks
A framework is not an architectural pattern, but rather a skeleton with a collection of “plug points” (also called hooks and slots) that enable it to be adapted to a specific problem domain.
Gamma et al note that:
Design patterns are more abstract than frameworks.
Design patterns are smaller architectural elements than frameworks
Design patterns are less specialized than frameworks

26. 1. What is Software Design? ----Characteristics of Software Design (6)
Components
A software components is a software element that
conforms to specific interactions and composition standards
can be independently deployed and composed without modification (according to a composition standard)
Common components
Local: ActiveX, OLE, JavaBean, COM
Network: COM+, EJB, CORBA, Web Service

27. The End Recommended Paper Next Lecture
Abstract from <Design Theory and Software Design>
Next Lecture
Software Architecture Design